A numerically controlled oscillator (NCO) or digitally controlled oscillator (DCO) is an electronic system for synthesizing a range of frequencies from a fixed timebase. A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time, to provide a stable clock signal for digital integrated circuits, to stabilize frequencies for transmitters/receivers, and the like.
Conventional systems and methods utilize either an analog voltage controlled crystal oscillators (VCXOs) in a phase-locked loop (PLL) or digitally controlled oscillators (DCOs) that include an analog to digital converter (ADC), so it can be used as replacement for an analog VCXO. This PLL is locked to the output from a timing module for system timing of transmitted data. Disadvantageously, both the VCXO and the DCO with ADC solutions require external analog filters (also known in the industry as deglitchers) in order to remove the instantaneous control signal from the oscillator, which requires a steady, noise-free, analog control voltage. This external filter adds cost, power, and potentially noise, as the control voltage is brought outside of the component.
Telecom and datacom systems are designed for multirate processing of various signals, such as, for example, OC-48, Optical Transport Unit level-1 (OTU-1), OC-192, Optical Transport Unit level-2 (OTU-2), Gigabit Ethernet, Fibre Channel, and the like. For multirate designs, multiple VCXOs are utilized with a multiplexer to engage the desired frequency, or a programmable DCO that includes an ADC, to act as VCXO replacement. Disadvantageously, the conventional systems and methods require multiple VCXOs or a DCO with an integrated ADC. Using multiple VCXOs requires a multiplexer, more printed circuit board area, and more power. Using a DCO with an integrated ADC is much higher cost. Furthermore, accurate and fast timing module (TM) failure detection is difficult, depending on the level of failure detection required.